W32JeefoPEFileInfector

Hunt Hypothesis

This detection identifies the W32/Jeefo malware family actively infecting Portable Executable (PE) files to establish persistence and potentially spread laterally across endpoints. Proactive hunting for this behavior in Azure Sentinel is essential because Jeefo’s low-severity file infection often evades standard signature-based defenses, requiring behavioral analysis to uncover early-stage compromises before they escalate into broader network incidents.

YARA Rule

rule W32JeefoPEFileInfector
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 55 89 E5 83 EC 08 83 C4 F4 6A 02 A1 C8 [3] FF D0 E8 [4] C9 C3 }

condition:
		$a0 at pe.entry_point
}

Deployment Notes

This YARA rule can be deployed in the following contexts:

• Microsoft Defender for Endpoint — Custom indicators / advanced hunting
• Email Gateway — Attachment scanning
• File Share Monitoring — Periodic scanning of shared drives
• YARA CLI — Manual threat hunting on endpoints

This rule contains 1 string patterns in its detection logic.

• Source Rule

False Positive Guidance

False Positive Scenarios for Rule: W32JeefoPEFileInfector

• Scenario 1: Automated Patch Deployment via Microsoft Endpoint Configuration Manager (SCCM)

  • Context: During nightly maintenance windows, SCCM pushes cumulative updates or application patches to endpoints. The deployment agent often extracts and modifies .exe or .dll files in the C:\Windows\Temp or ProgramData directories before installation, mimicking the PE file modification behavior of the Jeefo infector.
  • Suggested Filter/Exclusion: Exclude processes originating from Microsoft Endpoint Configuration Manager Client (ccmsetup.exe) and WUAHandler.exe. Additionally, add a path exclusion for files located within C:\Windows\CCM\Logs or C:\ProgramData\Microsoft\Endpoint Protection.

• Scenario 2: Antivirus Real-Time Scanning and Quarantine Operations

  • Context: Enterprise-grade AV solutions (e.g., CrowdStrike Falcon, Microsoft Defender for Endpoint) frequently scan and inject signatures into PE headers of executable files. When the AV engine updates its local database or performs a heuristic scan on a large batch of new downloads, it may temporarily modify file metadata, triggering the “PE Infector” logic.
  • Suggested Filter/Exclusion: Create an exclusion rule for processes running under C:\Program Files\CrowdStrike\ (e.g., csagent.exe) and C:\Program Files\Microsoft Defender Antivirus. Exclude events where the parent process is identified as the specific AV service.

• Scenario 3: Scheduled Software Updater Tasks (e.g., Adobe Creative Cloud or Java Update)

  • Context: Third-party software suites often run scheduled tasks that download and install updates in the background. For instance, the AdobeUpdate.exe process frequently unpacks new components into user